Record coverage of the initial seed corpus

In Fuzzbench, coverage reports do not start at the origin. Instead the Y-axis begins at the coverage achieved for the first data point (currently after 15 minutes). The initial seed corpus itself already achieves some initial coverage. It is easy to see that an arbitrary initial corpus achieves arbitrary initial coverage. Fuzzer performance should thus be measured relative to this initial coverage of the initial seed corpus. Otherwise, early saturation achieved by the fuzzer might be indistinguishable from saturation achieved by the seed corpus.